It isn't easy to directly answer how long birds can go without food, as several factors are worth considering. Birds are generally quite hardy little creatures with the ability to survive even in some of the harshest conditions on earth. But you cannot necessarily say that for all birds, as the relationship between food and birds differs, and some species find it difficult to cope under conditions they haven't adapted to. 
What factors determine how long a bird can survive without food?
As mentioned earlier, there are certain factors to consider first to answer this question correctly. What factors determine how long a bird can go without eating?
● Bird species: Different bird species are equipped differently to handle food shortages. A hummingbird, for example, can 'hibernate' to help survive food shortages in cold seasons.
● Size: Larger birds can go for long days without food than smaller ones.
● Temperature: Birds burn fewer calories during cold temperatures, helping them to survive longer without food.
● Fat storage: If the bird had been eating a lot of food days earlier, then it would build up enough fat stores to allow it to go for long periods. The fat stores provide energy to burn when there's no food.
● Last meal: Besides the fat stores, if a bird has recently filled its crop with a good food supply, it can go without eating for a longer period.
How long can birds go without food?
Okay, it's time to stop flocking around and answer the question. If you consider all the factors mentioned above, you can conclude that, on average, a small-sized bird can last for up to three days without food. Without body fat and in adverse conditions, a small bird will probably not survive more than 24 hours without food. 
Bird survival when resting without food
A small-sized bird can survive for about three to five days without food and a lowered metabolism state. Without body fat, its survival time will drop further.
Bird survival when active without food
Any typical bird with about 10% of its body weight made of fat can survive for up to three days while being active, much less without body fat. Again, this is only an estimate, and you cannot ignore the factors mentioned earlier.
Bird survival at normal temperatures without food
According to research conducted on European Starlings, roosting birds can go for about two days without food at average temperatures. However, the study didn't answer questions on body fat and other factors.
Bird survival in cold temperatures without food
Some studies conducted with small bird groups and lone birds indicated that when birds do not have sufficient energy to maintain normal body temperatures, they only last from 6 hours to a day without food for solitary birds. However, flock survival can help birds stay for three days without food and in cold temperatures.
Feeding time
The feeding pattern plays a crucial role in how long a bird can go without food. In most cases, thirty minutes after sunrise and around 5 or 6 in the evening are excellent times to feed your bird.